Modular communication systems, such as those used in spaceborne and airborne applications, typically employ highly compact and densified signal distribution/feed networks, such as multilayer stripline networks, to interconnect various components, such as RF signal processing (amplifier and impedance/phase control) circuits and beam-forming circuits for a phased array antenna. To minimize size and weight, it is common practice to stack multiple ones of such microstrip or stripline configured signal distribution networks as closely together as possible in a common support structure, such as in a laminated arrangement of printed circuits. A simplified illustration of such a laminated structure is diagrammatically illustrated in FIGS. 1 and 2 as patterns of conductors 1 and 2 and intermediate dielectric layers 3 (see FIG. 1), that are stacked together to form a three dimensional signal distribution architecture.
Because high frequency signal distribution networks, such as those employed for (RF) signalling applications in the hundreds of MHz or into the high GHz range, readily couple (radiate and receive) substantial electromagnetic energy in addition to that which is transmitted through the conductors of the networks, it is necessary to carefully configure and/or space such networks with respect to one another and adjacent system components. In FIGS. 1 and 2, this internal separation is shown by a horizontal spacing 4 and a vertical spacing corresponding to the thickness of an intermediate dielectric layer 3 between respective conductors 1 and 2. As far as the environment outside the network is concerned, the signal coupling problem is addressed by the use of (grounded) shielding layers, shown at 5 and 6 in FIG. 1.
However, within the multilayer structure itself, it can be expected that conductors of the respective networks will cross over or overlap one another at one or more locations, one of which is shown at 7 in FIG. 2. Because of the relatively reduced vertical separation between the conductors of the respective layers of the laminate, unwanted mutual coupling or cross-talk between the networks will occur at these cross-over points. A customary practice to solve this problem, diagrammatically illustrated in FIG. 3, is to insert a (copper) shielding layer 8 between each signal distribution layer. The shielding layer 8 is separated by dielectric layers 3 from conductor layers 1 and 2. On the top and bottom of the laminate are respective grounded shielding layers 5 and 6, lying atop respective dielectric layers 3.
Unfortunately, this not only adds to the weight of each intermediate metal layer, but substantially increases the overall thickness of the laminate, as additional dielectric layers 3 must be interposed between each shielding copper layer and a respective stripline layer. Moreover, the desire to keep such a laminate structure as thin as possible is countered by a trade-off between the thickness of the dielectric between the stripline and the ground layer and the lossiness of the stripline. Namely, because the effective impedance of the stripline is dependent upon its proximity to a ground layer, the thinner the dielectric, the narrower the line width of the stripline must be, in order to maintain a desired characteristic line impedance (e.g., fifty ohms, nominal). However, reducing the cross-section of the stripline increases its resistance and therefore its lossiness.